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Barton Springs Salamander -rangeland/pastureland

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This scenario has been parameterized to represent pesticide application on pastures, grassland, and rangeland in the Barton Springs Segment (BSS) of the Edwards Aquifer. Vegetation is generally dominated by grasses, forbs and shrubs. In North America, rangelands include the grasslands of the Great Plains including Midwestern United States to Manitoba, Saskatchewan, and Alberta in Canada. Other types of rangelands include US wetlands, Texas and Florida savannas, western US shrublands, Alaskan and Canadian tundra, Mexican deserts, and alpine meadows in mountain ranges throughout the North American continent (SRM 2004).

In the BSS, rangeland vegetation is a heterogeneous mixture of trees and grasses. Common tree species include: ash juniper (a nuisance species), oaks, hackberry and elms. Grass species including little blue stem, side oats gramma, indian grass, switch grass, king ranch bluestem (introduced) and kline grass (introduced) are typical. These areas are composed of approximately 60-65% trees and 30-35% grasses (Perez 2006). Although these landcovers contain a significant amount of tree cover, this "crop" was modeled as a field crop rather than an orchard in order to model a more conservative field.

Soils were selected based on their extent within the natural herbaceous land cover in BSS and the potential to yield high-end runoff and erosion. Based on a geospatial analysis of soils (USDA 2006) and land use data (USGS 2003) for natural herbaceous areas as well as conversations with local soil experts, Brackett soils were chosen to represent rangeland/pastureland areas in the BSS (Table 5). Brackett soils are found in both the contributing and recharge zones of the Edwards Aquifer and are the most common soil on which rangeland is located (USDA 2006; USGS 2003). Location of the Brackett soils was also cross-checked with aerial photography to ensure that the soil chosen coincided with natural herbaceous areas where pesticides may be applied. According to a local extension agent (Cris Perez), rangelands reside on a variety of soils; however Brackett is a common soil type of rangelands in the BSS.

The Brackett series was selected for this scenario because it is both highly representative of rangeland/pastureland areas in the BSS and because it represents the 90th percentile of vulnerability, drainage, erodibility, and slope. The Brackett series is a Hydrologic Group C soil which account for 23% of all soils in rangeland areas (Table 5). Hydrologic Group C soils account for approximately 49% of natural herbaceous soils in drainage. These soils are loamy, carbonatic, thermic, shallow Typic Haplustepts which consist of very shallow to shallow soils over bedrock. These well drained and moderately permeable soils formed in residuum over chalky limestone bedrock (USDA 2001). Brackett soils have a USLE K factor of 0.37 which includes the 90th percentile of these soils in erodibility (Table 5). Slopes range from 1 to 60 percent (Soil Survey Staff, 2006); however the most typical range for the Brackett series in rangeland areas is 1-8 percent (USDA 2006; USGS 2003). Soil parameters for the "Brackett-Rock outcrop-Comfort complex, 1 to 8 percent slopes" were selected from Soil Data Mart to parameterize this scenario since this soil type is the most extensive soil co-occuring with rangeland/pastureland within the Brackett series (USDA 2006; USGS 2003). Data from Hays County were selected since the majority of this landcover is located in this county.

The meteorological station selected for this scenario is located in Austin, Texas. This station is the closest available weather station that includes data required for PRZM.

Table 1.
PRZM 3.12 Climate and Time Parameters for Barton Springs, TX.
ParameterValue Source/Comments
Starting Date Jan. 1, 1961 Meteorological File from Austin, TX (W13958)
Ending Date Dec. 31, 1990 Meteorological File from Austin, TX (W13958)
Pan Evaporation Factor (PFAC) 0.69 PRZM Manual Figure 5.1 (EPA 1998).
Snowmelt Factor (SFAC) 0.36 PRZM Manual, Table 5.1 (EPA 1998).
Minimum Depth of Evaporation (ANETD) 25Mid point of range (20-30), PRZM Manual, Figure 5.2 (EPA 1998).

Table 2.
PRZM 3.12 Erosion and Landscape Parameters for Barton Springs - rangeland/pastureland.
ParameterValue Source/Comments
Method to Calculate Erosion (ERFLAG) 4 (MUSS) Default value.
USLE K Factor (USLEK) 0.37 tons EI-1* NRCS Soil Data Mart Database, Hays County, for Brackett-Rock outcrop-Comfort complex, 1 to 8 percent slopes. (http://soildatamart.nrcs.usda.gov/). Exit EPA Disclaimer
USLE LS Factor (USLELS) 0.69 Calculated according to Haan and Barfield (1978) equation: LS = ((λ/72.6)m)((430x2 + 30x + 0.43)/6.613), where λ = slope length, x = SLP/100 and m = constant. In this case, λ = 400 m (default value) and m = 0.4 (EPA 2004).
USLE P Factor (USLEP) 1 No contour plowing is expected (EPA 2004).
Field Area (AFIELD) 10 haDefault drainage area for standard ecological pond (EPA, 2004).
NRCS Hyetograph (IREG) 4 PRZM Manual, Figure 5.12 (EPA, 1998).
Slope (SLP) 4 %Brackett-Rock Outcrop-Comfort Complex Soil Slope range 1-8% (USDA 2006). Midpoint of slope range (EPA, 2004)
Hydraulic Length (HL) 356 m Default value for standard ecological pond (EPA, 2004)
Irrigation Flag (IRFLAG) 0 Cris Perez, NRCS - District Conservationist
Date: 3-16-06, Phone: 512-392-4050 x3

* EI = 100 ft-tons * in/ acre*hr

Table 3.
PRZM 3.12 Crop Parameters for Barton Springs - rangeland/pastureland.
ParameterValue Source/Comments
Initial Crop (INICRP) 1 Default value
Initial Surface Condition (ISCOND) 3 Cris Perez, NRCS - District Conservationist
Date: 3-16-06, Phone: 512-392-4050 x3
Number of Different Crops (NDC) 1 Set to number of crops in simulation. Default value.
Number of Cropping Periods (NCPDS) 30Set to weather data in meteorological file: Austin, TX (W13958).
Maximum rainfall interception storage of crop (CINTCP) 0.2 At their maximum growth, grasses may intercept as much as 20% of gross precipitation during individual storms (Dunne and Leopold, 1978).
Maximum Active Root Depth (AMXDR) 43 cm Cris Perez, NRCS - District Conservationist; Date: 3-16-06, Phone: 512-392-4050 x3
Root depth depends upon the soil depth. In creek beds, grass roots will grow 3-4'. On rolling hills, they will grow 6-8". Therefore, this value set to CORED.
Maximum Canopy Coverage (COVMAX) 97% Cris Perez, NRCS - District Conservationist
Date: 3-16-06, Phone: 512-392-4050 x3
Soil Surface Condition After Harvest (ICNAH) 3 Cris Perez, NRCS - District Conservationist
Date: 3-16-06, Phone: 512-392-4050 x3
Date of Crop Emergence
(EMD, EMM, IYREM)
01/03/61 Cris Perez, NRCS - District Conservationist
Date: 3-16-06, Phone: 512-392-4050 x3

Plants emerge from late February-March. They mature mid June. They go dormant after the first frost, which occurs in November.

Date of Crop Maturity
(MAD, MAM, IYRMAT)
15/06/61
Date of Crop Harvest
(HAD, HAM, IYRHAR)
15/11/61
Maximum Dry Weight (WFMAX) 0.0Not used in scenario.
Maximum Canopy Height (HTMAX) 122 Little bluestem (2-4') is a typical range plant for this soil (USDA 2006) in this region (Cris Perez, NRCS). Height data from http://texnat.tamu.edu/cmplants/B-182/main.htm. Exit EPA Disclaimer
SCS Curve Number (CN) 87, 83, 86 Gleams Manual Table H-4, pasture/range, non-CNT, hydrologic group C, poor condition (USDA, 2000)
Manning's N Value (MNGN) 0.110 San Antonio Pasture, warm season (I93PWPWN). This file incorporates no tillage and has a cover code (2) representing first year grass, pasture or hay crops.
USLE C Factor (USLEC) 0.004 San Antonio Pasture, warm season (I93PWPWN).

Table 4.
PRZM 3.12 Brackett-Rock Outcrop-Comfort Complex Soil Parameters for Barton Springs - rangeland/pastureland.
ParameterValue Source/Comments
Total Soil Depth (CORED) 43 cm NRCS Soil Data Mart Database, Hays County, for Brackett-Rock outcrop-Comfort complex, 1 to 8 percent slopes. (http://soildatamart.nrcs.usda.gov/). Exit EPA Disclaimer

According to an extension agent (Cris Perez), rangelands reside on a variety of soils. Brackett is a common soil type of rangelands in this area.

Number of Horizons (NHORIZ) 3 Additional data were listed for a 4th HORIZN. However, these were not included in this soil profile since the 4th HORIZN is composed of bedrock.
Horizon Thickness (THKNS)
  • 10 cm (HORIZN =1)
  • 5 cm (HORIZN =2)
  • 28 cm (HORIZN =3)
Bulk Density (BD)
  • 1.4 g/cm3 (HORIZN =1)
  • 1.4 g/cm3 (HORIZN =2)
  • 1.4 g/cm3 (HORIZN =3)
Initial Water Content (THETO)
  • 0.28 cm3/cm3 (HORIZN =1)
  • 0.28 cm3/cm3 (HORIZN =2)
  • 0.251 cm3/cm3 (HORIZN =3)
Compartment Thickness (DPN)
  • 0.1 cm (HORIZN =1)
  • 5 cm (HORIZN =2)
  • 4 cm (HORIZN =3)
PRZM Scenario Guidance (2004).
Field Capacity (THEFC)
  • 0.28 cm3/cm3 (HORIZN =1)
  • 0.28 cm3/cm3 (HORIZN =2)
  • 0.251 cm3/cm3 (HORIZN =3)
Wilting Point (THEWP)
  • 0.164 cm3/cm3 (HORIZN =1)
  • 0.164 cm3/cm3 (HORIZN =2)
  • 0.142 cm3/cm3 (HORIZN =3)
Organic Carbon Content (OC)
  • 1.16 % (HORIZN =1)
  • 1.16 % (HORIZN =2)
  • 0.73 % (HORIZN =3)
Adjusted using the relationship % OC = % Organic Matter/1.724 (Doucette 2000).

Table 5.
Soils co-located with natural herbaceous areas of the Barton Spring Segment based on USDA 2006 soils data and USGS 2003 land use data. Bold font indicates a benchmark soil.
SoilTotal Acreage% Area Drainage ClassKFSlope (%)pHOM (%)Sand (%) Silt (%) Clay (%)
Brackett 8,54022.8%C0.371 - 128234 38 28
Rumple 5,941 15.8% C 0.32 1 - 8 7 2 34 37 30
Doss 4,001 10.7% D 0.32 1 - 5 8 2 7 49 44
Real 2,518 6.7% D 0.28 1 - 8 8 6 36 34 31
Comfort 1,654 4.4% D 0.32 1 - 8 8 6 28 29 43
Volente 1,625 4.3% C 0.32 1 - 8 8 3 7 54 39
Bolar 1,591 4.2% C 0.32 1 - 3 8 2 34 37 30
Sunev 1,413 3.8% B 0.32 0 - 1 8 2 18 52 30
Krum 1,404 3.7% D 0.32 0 - 1 8 2 26 29 45
Purves 961 2.6% D 0.32 1 - 5 8 3 23 29 48
Denton 907 2.4% D 0.32 1 - 3 8 3 6 48 46
Austin 867 2.3% C 0.32 1 - 3 8 3 7 48 45
Tarpley 768 2.0% D 0.32 1 - 3 7 3 30 30 40
Lewisville 753 2.0% B 0.32 0 - 1 8 2 8 51 41
Tarrant 596 1.6% D 0.32 5 - 18 8 5 22 28 50
Speck 569 1.5% D 0.32 1 - 3 7 2 34 37 30
Crawford 485 1.3% D 0.32 0 - 1 7 2 22 28 50
Houston Black 406 1.1% D 0.32 0 - 1 8 3 17 28 55
Anhalt 353 0.9% D 0.32 1 - 3 7 3 26 29 45
Gruene 341 0.9% D 0.28 1 - 5 8 2 28 29 43
Heiden 295 0.8% D 0.32 1 - 3 8 3 22 28 50
Castephen 207 0.6% C 0.32 3 - 5 8 2 34 32 34
Alluvial land 196 0.5% A 0.15 0 - 1 8 1 90 0 5
San Saba 187 0.5% D 0.32 1 - 2 8 3 18 29 53
Branyon 160 0.4% D 0.32 0 - 1 8 3 22 28 50
Seawillow 141 0.4% B 0.32 1 - 3 8 1 35 34 31
Tinn 99 0.3% D 0.32 0 - 1 83 22 28 50
Medlin 96 0.3% D 0.32 1 - 8 82 22 28 50
Oakalla 88 0.2% B 0.32 0 - 1 84 18 48 34
Boerne 73 0.2% B 0.28 1 - 3 81 65 20 16
Orif 61 0.2% A 0.28 0 - 1 82 82 9 9
Eckrant 57 0.2% D 0.32 8 - 40 87 22 28 50
Urban land 50 0.1% D 0.00 0 - 6 00 0 0 0
Altoga 46 0.1% C 0.32 1 - 3 81 7 48 45
Patrick 45 0.1% B 0.32 2 - 5 82 28 29 43
Eddy 15 0.0% C 0.32 1 - 3 81 38 36 26
Hardeman 2 0.0% B 0.24 3 - 12 81 66 20 14
Travis 1 0.0% C 0.24 1 - 8 71 66 19 15
Ferris 0 0.0% D 0.32 8 - 20 81 18 29 53
Gaddy 0 0.0% A 0.17 0 - 1 80 84 7 10

Sensitive Parameter Uncertainties

Crop Parameters

As discussed above, rangeland vegetation is a heterogeneous mixture of trees and grasses. For the purposes of modeling, it was necessary to select crop specific parameters (Table 3) that are representative of rangeland plants. In order to model areas that would be more likely to be subject to pesticide applications and susceptible to runoff, grassy areas were selected for the conceptual model of this scenario, rather than tree areas. This decision was necessary for the selection of several sensitive parameters, including CN, USLEC and Manning's n values.

For USLEC and Manning's N values, the file for San Antonio Pasture, warm season (I93PWPWN) was selected. This file incorporates no tillage (NT) and has a cover code representing first year grass, pasture or hay crops (2). This file was selected because it models a tillage system and crop that seem appropriate for a rangeland or pastureland. A RUSLE data file also exists for San Antonio range (I93RARAN) but incorporates a cover code (6) which represents no cover (0-7% residue cover on soil surface during critical period) and is commonly used with conventionally tilled crops. This cover code did not seem appropriate to model a pastureland which would have residue during the critical period. Therefore, the file for range was not selected.

References

Dunne, T., and L. Leopold. 1978. Water in Environmental Planning. W.H. Freeman an Company, New York. 818 pp.

EPA. 1998. Carsel, R.F., J.C. Imhoff, P.R. Hummel, J.M. Cheplick, and A.S. Donigian, Jr. PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in the Crop Root and Unsaturated Soil Zones: Users Manual for Release 3.0. National Exposure Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.

EPA. 2004. Pesticide Root Zone Model (PRZM) Field and Orchard Crop Scenarios: Guidance for Selecting Field Crop and Orchard Scenario Input Parameters. November 15, 2001; Revisions July 2004.

Haan, C.T. and B.J. Barfield. 1978. Hydrology and Sedimentology of Surface Mined Lands. Office of Continuing Education and Extension, College of Engineering, University of Kentucky, Lexington, Kentucky 40506. 286 p.

Perez, C. 2006. NRCS - District Conservationist. Personal conversations. March 17 & 20, 2006.

Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. 2006. Official Soil Series Descriptions [Online WWW]. Available URL: http://soils.usda.gov/technical/classification/osd/index.html [Accessed 6 March 2006]. Exit EPA Disclaimer

SRM. 2004. Society for Range Management. "What are rangelands?" http://rangelandswest.org/whatarerangelands.html. Exit EPA Disclaimer Last Revised: 23 March 2004. Accessed 14 March 2006.

USDA. 2000. Knisel, W.G., and Davis, F.M., 2000, GLEAMS: Groundwater Loading Effects of Agricultural Management Systems, Version 3.0: Agricultural Research Service, U.S. Department of Agriculture, Publication No. SEWRL-WGK/FMD-050199, 191 p.

USDA. 2001. Official Series Description. Brackett Series. Information from the website: https://soilseries.sc.egov.usda.gov/OSD_Docs/B/BRACKETT.html.

USDA. 2006. Soil Survey Areas of Hays Counties, Texas. U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS), Soil Data Mart. March 1, 2006. Online at: http://soildatamart.nrcs.usda.gov. Exit EPA Disclaimer

USGS, National Mapping Division, Rocky Mountain Mapping Center. 2003. Edwards Aquifer Land Use / Land Cover. Denver, Colorado.


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